Quote Originally Posted by harry mac View Post
Mik, for as long as I can remember, British publications have put the devisor at 450240. American sources I've seen all quote 450400. I don't know why there's a fifference.
Do we know what figure the police/phorensic service use when/if they have to test an airgun?
The divisor is the result of the acceleration from gravity ("g") and the number of grains in a pound, and the constant "2". The number of grains in a pound and the value "2" are fixed values. The differences arise from small differences in the value assumed for "g". "g" is not a constant all over the surface of the earth, it varies with altitude, underlying geology, and proximity to the equator, so any value applied universally will likely be an approximation to the value of "g" at your exact location. For perfect accuracy the forensic service would need to ascertain the exact value of "g" at the test location and adjust the calculation accordingly, but the differences attributable to variations in local "g" are very small (approximately .03% between the values you quote) and much smaller than the other measurement errors. To be credible, a forensic result for muzzle energy would also have to specify the maximum error in the given figure, e.g. 11.98 ft-lbf +_ 0.1%